Adapting the “Simplest Pressurized Storage System” to my existing system.

[Shelterman]

After much deliberation, I have decided to order a Biomass 60 and install my system this summer. I can't begin to tell you how much this form has helped me in my decision making process. However, I need a little help in integrating the new wood system with my existing hydronic system.

First some basic information... I will be heating both a 21' x 60' greenhouse (triple wall insulated, polycarbonate glazing) and my 4500 sq ft home. For storage, I have a 500 gallon propane tank and am currently trying to locate another 500 or 1000 gallon tank. The Biomass 60, the storage tanks, and a backup propane boiler will all be located in an outbuilding approximately 110’ from my house and the greenhouse will be approximately 30’ from the outbuilding. My plan is to plumb the system along the lines of the “Simplest Pressurized Storage System” and integrate it with my existing system.

My house currently has two separate forced air hydronic heating zones which are completely independent of each other, each with separate thermostats and circulators. What I would like to do is plumb the system with the “Simplest Pressurized Storage System” design, but instead of using zone valves on all three zones (the third zone being the greenhouse), I'd like to use my existing circulators and thermostats in the house, and use a zone valve or circulator for the greenhouse zone.

I guess my first question is should I plumb the load lines (a 1” pex circuit for each zone… making a total of four 1” buried lines) so that they bypass the load circulator or should I wire the house relay to the load circulator relay so that the load circulator operates in the series with either one of the house circulators? Or should I just do away with the load circulator and zone valve completely, and operate each zone with its own circulator?

One problem I see with using circulators instead of zone valves is that when two or more of the zone circulators are calling for heat, the propane and wood circulators won't be able to supply the needed flow causing undesired flow from the storage tanks. If this a real big problem, perhaps I be should looking at a “primary/secondary” type system instead…would it be more adaptable to my existing system?

If anyone can give me a push in the right direction on this I would really appreciate it

 

[ewdudley]

Here's my suggestion, somewhat less simple, but it helps get the flow control you need to address the problems you've noted.

Treat the storage tank as a buffer, which it is. The wood boiler is allowed to attempt to fill the storage completely, but the LP boiler, when used, would be set up to cycle off anytime a layer of hot water is established at the top of the tank.

Independently, the greenhouse circuit can draw from storage and/or boiler flow any time it needs hot water and there is water hot enough to be useful for the greenhouse circuit.

On the house side, add a buffer tank, a.k.a. hydraulic separator tank, on the order of 30 gallons or more. An independent circ pump is controlled on and off or at variable speed to maintain a supply of hot water in the buffer tank any time there is water hot enough available. The independence of a transfer-to-house pump makes possible minimizing return temperature to storage to maintain maximum stratification. And in the off-season the buffer tank allows the transfer pump to operate in a batch mode while the DHW circuit draws off heat at its own rate.

Then the house loads draw from the buffer tank according to demand. I've drawn three circuits with three pumps, each tuned to its purpose, but a variety of options would be viable. The key point is that the house loads draw from the buffer independently from the flow that maintains the buffer.

 

[Shelterman]

Eliot,

Thank you for your quick response and your diagram. The diagram really helps me understand your solution.

I haven't had a chance to put a lot ofthought into your design, but my first thoughts are that it not only resolves the issue that I outlined, but it also resolves an issue or two but I didn't mention. One being DHW... this makes supplying the DHW needs of my house much easier, and eliminating the long underground line back to the outbuilding. This also eliminates the expense of the additional underground Pex line, providing that a single 1- 1.25" line can fully supply the BTU needs to my house during the coldest of weather (~0 to -5 deg.)

I will put some more thought into this idea throughout the day.

David

 

[ewdudley]

This stuff is probably in the stickies if you dig it out, but at any rate studying Siegenthaler's stuff is worthwhile:

http://tinyurl.com/yzyrjyg

http://tinyurl.com/yexddfr

These articles suggest a possibly simpler, less expensive, and decidedly more elegant solution.

Instead of the buffer tank, the 'transfer pump' and the three circ pumps on the house side, substitute a Grundfos Alpha or Wilo Stratos ECM constant-pressure pump to service the house loads and use zone valves for the heating zones in the house and the DHW.

These pumps are very slick. You set the desired pressure and as zones come on and off the pump responds with more or less flow to maintain pressure.

Each of the main loads and the DHW might need an adjustable flow-control valve in series to make possible 'impedance matching' for the three circuits in order to achieve optimal deltaT in each circuit.

These pumps allow the operator to adjust the pressure setting to suit the season in a manual outdoor reset mode. Versions of these pumps also exist that will accept an external control signal interface that would make them compatible with Tekmar controls for automatic outdoor reset control.

 

[Shelterman]

Ok. I've just started to digest the info at the Pmengineer.com website. I've just begun to grasp basic hydronics, so this is a new twist for me...

So if I'm understanding you right, and ECM pump will it adjust its GPM output based on the number of zones in my house that are open, and eliminating the buffer tank in the house. Right?

One of the things that I was wanting to accomplish is to keep my existing hydronic system as intact as possible in case I need to revert back to it in the future. Will a ECM pump work with my existing Grundfos pumps that are currently in place or will it work only with zone valves?

 

[ewdudley]

Ok. I've just started to digest the info at the Pmengineer.com website. I've just begun to grasp basic hydronics, so this is a new twist for me...

So if I'm understanding you right, and ECM pump will it adjust its GPM output based on the number of zones in my house that are open, and eliminating the buffer tank in the house. Right?

One of the things that I was wanting to accomplish is to keep my existing hydronic system as intact as possible in case I need to revert back to it in the future. Will a ECM pump work with my existing Grundfos pumps that are currently in place or will it work only with zone valves?

For the constant-pressure ECM pump to work the individual zones have to shut off completely so that the pump can 'see' that no flow is required for that zone. To keep your existing system 'intact' you'd need zone valve assemblies that would bolt-in in place of your zone circulator assemblies. Should be possible, but maybe not, there's only six inches or so to work with and it could get too tight for two flanges and a zone valve.

At any rate, to back up a bit to the original question, to adapt the SPSS to your situation:

I guess my first question is should I plumb the load lines (a 1” pex circuit for each zone… making a total of four 1” buried lines) so that they bypass the load circulator or should I wire the house relay to the load circulator relay so that the load circulator operates in the series with either one of the house circulators? Or should I just do away with the load circulator and zone valve completely, and operate each zone with its own circulator?

You only need a single sufficiently sized supply and return pipe to the house. Each of your existing house zones could simply draw in parallel through the single pipe from the heating plant. I don't think you could have an in-series 'Load Circ' as shown in the SPSS schematic because it might be a problem to force some flow through both house zones whether their pumps were running or not. If your calculations show that more head is required to pull sufficient flow all the way from storage for either of the zones then you'd need to replace the pump for that zone with a different type of pump.

One problem I see with using circulators instead of zone valves is that when two or more of the zone circulators are calling for heat, the propane and wood circulators won’t be able to supply the needed flow causing undesired flow from the storage tanks. If this a real big problem, perhaps I be should looking at a “primary/secondary” type system instead…would it be more adaptable to my existing system?

If your storage is depleted and the heat sources can't keep up temporarily, then you simply need an aquastat that prevents circulation until hot water is available. Also you could add a priority zone controller box or some roll-your-own relay logic to insure that any capacity that is available goes to the highest priority demand.

The primary goal of each of my suggestions above is to simplify the task of insuring that water returning to storage is as fully depleted as possible. What to be avoided is to bring return water back to storage that is any warmer than necessary. With the buffer tank the idea is to adjust the return temperature from the buffer tank until it is just high enough to have the house system perform satisfactorily. With constant-pressure ECM pump the idea is to adjust the pressure setting so that the flow to the house is just enough to supply each of the loads when it calls for heat.

For DHW the buffer tank has the advantage of making possible batch transfer of hot water to the house during the off season. The idea is that the pipe to the house isn't kept as hot for as long as it would be to support intermittent and piddling flows for summertime DHW. Then again the arithmetic would probably show that this is not something to worry much about.

So:

Option A: Single pipe to the house. Each house zone draws off the single pipe. Study each house zone and replace pump if necessary to insure that flow is just right to supply sufficient heat for each zone while minimizing return temperature. Likewise DHW has its own circ, study the flow requirements and make sure pump is no bigger than necessary for supplying the DHW heat exchanger. Add in-series flow control tuning valves to zones if necessary.

Option B: Same as option A, but insert buffer tank that is supplied by an independent circ pump and control. Since the buffer tank assumes the responsibility of controlling return temperature to storage, the sizing of pumps on the house side becomes less critical, but still needs to be looked-at. Batch-mode summer DHW becomes possible, assuming that batch-mode DHW is worthwhile to begin with.

Option C: Single pipe to the house same as Option A or Option B, but replace all circs with zone valves and circulate to the house with a single ECM constant-pressure pump. Tune system for minimum return temperature to storage by adjusting pressure setting. Add in-series flow control tuning valves to zones if necessary. Depending on whether an ECM pump with proper dynamic range is available, and assuming the flow resistance of the greenhouse circuit can be made compatible, the ECM pump could supply all loads for the entire system.
.

 

[Shelterman]

Eliot,

I don't want any more time to to pass by without thanking you for your assistance in helping me with my system layout. I am leaning toward your Option A as it seems to be the easiest for me to implement. My only hesitation with the proposed single pipe to the house is the fact that during the coldest part of the winter my home's heating demand will be at least 160,000 BTU/hour so 1" pex just won't cut it. And as the cost 1 1/2" pex is almost 3 times the cost of 1" pex I suppose economics is a consideration.

I located a 1000 gal. propane tank today so I am very close to getting the work started on this project. I figure with no more spare time than I have to work on projects like this that I will still be pressed to get everything completed before next winter.

I plan on documenting my work as it progresses, complete with pictures and will post them on this form sometime in the future.

David

 

[ewdudley]

I don't want any more time to to pass by without thanking you for your assistance in helping me with my system layout.

I'm merely trying to complete this portion of my own design, and writing stuff down is always the best way to clarify one's thinking, so I'm glad it's clarified to the point of being helpful to anyone.

I am leaning toward your Option A as it seems to be the easiest for me to implement.

I think I would be too, especially since you're driving hydronic-to-air heat exchangers as opposed to radiant emitters. The only real disadvantage is the difficulty of selecting the right pumps to get the optimal flow for maximum deltaT versus adequate btu delivery. I would definitely look into some sort of adjustable flow control device in each of the house zones so you can limit the flow to be no more than necessary once you've found a pump with the right pump curve for pulling all the way from storage.

My only hesitation with the proposed single pipe to the house is the fact that during the coldest part of the winter my home's heating demand will be at least 160,000 BTU/hour so 1" pex just won't cut it. And as the cost 1 1/2" pex is almost 3 times the cost of 1" pex I suppose economics is a consideration.

Pipe size from heating plant to house has been discussed an awful lot on the forum, but nobody sums it up better than the Taco TD10 document. It is called out in the forums tidbits area and is available from Taco of course: http://www.taco-hvac.com/uploads/FileLibrary/SelectingCirculators.pdf

But be especially aware that the Taco 4.0 feet per second water velocity limitation is dictated by water noise considerations, which is not a factor for underground pipe. For underground pipe you can go up to 8.0 feet per second and thereby double the flow rate. However the power required to shove water down a pipe varies as gpm to the 1.75 power, so system life-cycle electricity consumption over the long haul is a consideration.

I suspect a lot of guys are putting in bigger pipe than necessary, and naturally so because you sure don't want to go through the whole expense an hassle to find out you cheaped-out on the pipe itself.

Nonetheless you could save a lot of money on the pipe if you really sharpen your pencil and make sure you're not making it bigger than necessary.

If you can show that the 160,000 btu per hour number is actually a lot higher than necessary, and if you can show that your heat exchangers can deliver enough heat with gpm X and deltaT Y, and if you can show that the power required to pump the sum of the gpms is not too expensive in terms of kilowatt-hours over the life-cycle of the system, then it's quite possible that single nominal 1.25" inch pex would do the job.